The present invention relates in general to the production of a metal sheet with plural holes therein. More particularly, the invention relates to metal sheets suitable for use as a part of a microwave oven door.
A microwave oven door comprises a transparent glass plate and a thin metal sheet of generally rectangular shape with plural small holes in a central "window" portion thereof. Food cooking in a cooking cabinet of the oven can be viewed through the glass plate and the holes in the metal sheet. If the holes are made sufficiently small, microwaves do not pass through the holes to the outside of the cooking cabinet. The metal sheet reinforces the door. Surrounding the window portion of the metal sheet there is an edge portion which has a flange formed by press molding.
A typical way to manufacture such a metal sheet is to first punch small holes in a portion of the metal sheet having a thickness of, for example, 0.6 mm. After punching the holes, the edge portion of the sheet is press molded to form the flange.
The user wants to be able to easily see the food in the cooking cabinet through the window portion of the metal sheet. Safety requires that the holes be sufficiently small to block microwaves. However, in order to see the food being cooked easily, it is desirable to make the holes as large as possible.
A so called "opening ratio" represents the total area of the holes divided by the area of the window portion of the metal sheet. To promote visibility, it is desirable to have a large opening ratio.
However, if the diameter of the holes is made large, a width of metal separating the holes from each other becomes narrow. As a result, a strain strength of the window portion becomes low. When the flange is formed by press molding, the central portion of the metal sheet is strained to the outside of the metal sheet. The corner area of the central portion is particularly strained. If the strain strength of the metal sheet is insufficient because the holes are large, the metal sheet will break during press molding.
In general, to prevent such breaking, the diameter of holes punched in the window portion of the metal sheet is made small. For example a pitch between two holes is usually 1.6 mm and the hole diameter is 1.2 mm. These typical pitch and diameter measurements cause the opening ratio to be about 51%, which is not sufficient for good visibility through the door.
Another approach to solving this problem includes a welding operation. A metal sheet has a central window portion with plural holes punched therein. A separately manufactured frame is welded to the metal sheet. The opening ratio can be made sufficient for good visibility, however the welding process is complicated and more materials are required. This makes the production cost high and this alternative unacceptable. Also, the strength of the metal sheet is reduced during the manufacturing process.